An automatic transmission incorporated in a vehicle such as an automobile includes a planetary gear set and a plurality of friction fastening elements such as a multi-plate clutch and a multi-plate brake. As the friction fastening elements are selectively fastened according to the operation state of an engine, it is possible to automatically shift to a predetermined gear position. The friction fastening element includes a plurality of friction plates disposed with a clearance therebetween and a piston pressing the friction plates. The piston is moved between a fastening position where the friction plates are pressed to be fastened to each other and a release position where pressing of the friction plates is released and thus the friction plates are in a released state.
Patent Literature 1 discloses an automatic transmission in which a fastening hydraulic chamber is disposed on one side in an axial direction of a piston and a centrifugal hydraulic chamber (release hydraulic chamber) is disposed on the other side, and a through-hole through which the fastening hydraulic chamber communicates with the centrifugal hydraulic chamber is formed in the piston. A predetermined hydraulic pressure is supplied from different oil passages to the fastening hydraulic chamber and the centrifugal hydraulic chamber. A pressure regulator valve is assembled in the oil passage for the centrifugal hydraulic chamber.
It is necessary in the automatic transmission to shorten a fastening control time while reducing a fastening shock applied to the friction fastening elements when being switched from a released state to a fastened state. To shorten the fastening control time and reduce the fastening shock at the same time, it is necessary to cause a large flow rate of oil to flow at a precharge hydraulic pressure or the like for the purpose of shortening the fastening control time, and at the same time, to reduce the flow rate (reduce the hydraulic pressure) of oil immediately before a stroke is completed for the purpose of reducing the fastening shock. In this case, precise flow rate control must be executed, so that hydraulic pressure control tends to be complicated. Consequently, it takes a long time to execute fastening control for fastening the friction fastening elements, thus reducing response of the friction fastening elements.